In order to reduce power losses and efficiency losses of gas turbines and combined cycle power plants with carbon dioxide separation, various ways have been proposed of increasing the carbon dioxide partial pressure before separation.
Exhaust gas recirculation is a technology which can be used for different purposes in gas turbines. Thus, it can be used for controlling the NOx emission by the provision of an intake gas with reduced reactivity, for example, that is to say usually with oxygen content which is reduced compared with fresh air, or used for reducing the exhaust gas volume for the carbon dioxide separation. During exhaust gas recirculation in a gas turbine, a significant proportion of the exhaust gas is branched from the overall exhaust gas flow and, for example, after cooling and, if necessary, after scrubbing, is fed again to the intake mass flow of the turbine or of the compressor of the gas turbine. To this end, the recirculated exhaust gas flow is mixed with fresh air and this mixture is then fed to the compressor. A corresponding power plant with a gas turbine and exhaust gas recirculation is known from WO2010/072710, for example.
In an HRSG (heat recovery steam generator or waste heat boiler) and in exhaust gas ducts downstream of the gas turbine, fuel residues can accumulate when the plant is being shut down. These fuel residues are removed from the volumes in question by means of so-called boiler purging before a restart of the plant. The aim of this safety measure is to free the exhaust gas ducts and the HRSG of residual fuels and explosive gas mixtures and to avoid potential explosions during startup. The boiler purging is carried out by driving the shaft train, by means of a starting device, at a low speed and with a low mass flow, as a result of which residual fuels are driven out of the plant via the exhaust stack.
In the case of gas turbines with exhaust gas recirculation, residual fuels can also accumulate in the exhaust gas recirculation lines and auxiliary systems, such as recirculation coolers, water separators, etc., of the exhaust gas recirculation system when the plant is being shut down. Before a restart, or before these lines and auxiliary systems are accessible for inspection or repair, these must also be purged, for example with fresh air.
The exhaust gas recirculation lines may be purged opposite to the flow direction during normal operation. Therefore, the purge air can be discharged via the normal exhaust stack. It is ensured, moreover, that combustible gas residues which possibly exist do not find their way from the boiler into the exhaust gas recirculation lines, which would result in the purging duration having to be further extended or even a cycle having to be created.
A system and method for the purging of exhaust gas recirculation lines are proposed in EP2060772. Additional flaps and, for example, an additional fan are proposed in EP2060772 for exhaust gas recirculation to allow the exhaust gas recirculation line to be purged. The large number of flaps and the additional fan lead to increased costs and increased power demand during the purging process, as a result of which the competitiveness of such plants with CO2 separation can be compromised.